For more than 200 years the conventional papermaking process is based on a filtration process of aqueous suspensions of woodfibers. Due to the large flocculation tendency, which can cause optical inhomogenities in the final paper structure, typically low consistencies of about 0.5-2% (by weight) woodfibers are used in paper furnishes. A large part of the production energy is consumed by the drying process, as water forms typically about 50% (by weight) of the wet web structure after filtration and pressing, and has to be evaporated in the drying section of the process.
Paper-like products have also been manufactured from non-cellulosic raw materials (e.g. ViaStone or FiberStone). Such products may consist of 80% calcium carbonate and 20% synthetic polymer resin, for example. By such materials, water consumption can be reduced or even avoided.
In certain applications, woodfibers have been replaced with nanocellulose as the raw material. This enables opportunities for new products, and new papermaking processes.
Henriksson et al, Cellulose Nanopaper Structures of High Toughness, Biomacromolecules, 2008, 9 (6), 1579-1585 discloses a porous paper comprising a network of cellulose nanofibrils. The preparation of the paper starts from nanofibril-water suspension, where the water is removed so that a cellulose nanofibril network is formed. First, a 0.2% (by weight) stirred water suspension is vacuum filtrated in a filter funnel. The wet films obtained is dried under heat and pressure. Porosity of the product was increased by exchanging the water as a solvent for methanol, ethanol or acetone before drying.
US 2007/0207692 discloses a nonwoven transparent or semitransparent highly porous fabric containing microfibrillated cellulose. The fabric can be obtained by a similar process as in the abovementioned article of Henriksson et al. by forming a web from aqueous suspension of microfibrillated cellulose, exchanging the water solvent for organic solvent and drying. According to the examples, the consistency of the aqueous suspension is 0.1% (by weight) before web-forming. Both the abovementioned methods utilize nanocellulose fibers that are smaller in size than the cellulose fibers (wood fibers) used in conventional paper making. Sheets manufactured from nanocellulose fibers are reported to have high toughness and strength. However, due to their transparency and/or porosity they are not very suitable as such for printing purposes, for example.
In addition, there is a need for more efficient methods of manufacturing paper, paperboard or the like products from nanocellulose.